Valgma landscape

An Analysis of Vegetation Restoration on OpencastOil Shale Mines in EstoniaMargus Pensa,1,2,5 Arne Sellin,3 Aarne Luud,1 and Ingo Valgma4

Abstract

We compared four types of 30-year-old forest standsgrowing on spoil of opencast oil shale mines in Estonia.The stand types were: (1) natural stands formed byspontaneous succession, and plantations of (2) Pinussylvestris (Scots pine), (3) Betula pendula (silver birch),and (4) Alnus glutinosa (European black alder). In allstands we measured properties of the tree layer (speciesrichness, stand density, and volume of growing stock),understory (density and species richness of shrubs and treesaplings), and ground vegetation (aboveground biomass,species richness, and species diversity). The tree layer wasmost diverse though sparse in the natural stands. Under-story species richness per 100-m2 plot was highest in thenatural stand, but total stand richness was equal in the

natural and alder stands, which were higher than the birchand pine stands. The understory sapling density was lowerthan 50 saplings/100m2 in the plantations, while it variedbetween 50 and 180 saplings/100m2 in the natural stands.Growing stock volume was the least in natural stands andgreatest in birch stands. The aboveground biomass ofground vegetation was highest in alder stands and lowest inthe pine stands. We can conclude that spontaneoussuccession promotes establishment of diverse vegetation.In plantations the establishment of diverse ground vegeta-tion depends on planted tree species.

Key words: Alnus glutinosa (L) Gaertn., Betula pendulaRoth, forest plantation, opencast mine, Pinus sylvestris L.,restoration, spontaneous succession.

Introduction

Thedestruction of ecosystem throughmining forminerals andother activities tomeet industry demands has been an intrinsicpart of modern development. Further need for mineralresources will accelerate degradation of natural habitats,which will result in reduced biodiversity (Singh et al. 2002).In the second half of the twentieth century scientists andengineers were presented with many challenges to achieverestoration, but the less difficult goal of reclamation wasmore often practiced on human-disturbed areas around theworld (definitions as in www.ser.org). As the utilization ofnatural resources continues and opportunities to restoreecosystems damaged by human activities become more com-mon, restoration is playing an increasingly important rolein environmental protection (Prach et al. 2001). The publicresponds emotionally to lands degraded by mining activitiesand associates mining with land that has been left devoidof all topsoil, all vegetation, and any hope of regeneration inthe short to mid time scale.The Convention on Biological Diversity (1992) signed

by most states of the world in Rio de Janeiro calls for

ecologically sound restoration of degraded ecosystems asmeasures to promote the recovery of local biodiversity.Governments have therefore frequently given resources toreestablish vegetation on degraded lands, in anticipationthat this will lead to restoration of the preexisting ecologicalstate and may add economic value to the degraded lands(Hunter et al. 1998). The traditional approach to reclama-tion has been to sow grass and legumes and plant trees tominimize financial and human resource expenditures. Land-scape engineers and foresters often establish a low-diversityplant cover or use monospecific plantations of exotic species(Hunter et al. 1998; Rebele & Lehmann 2002). Althoughplantations can play a key role in restoring forest eco-systems and achieving short-term socioeconomic goalsby protecting the soil surface from erosion, catalyzingdevelopment of native forests, and accelerating the recoveryof genetic diversity (Singh et al. 2002), spontaneous vegeta-tion succession, or natural recovery, as an alternativeapproach to restoration or reclamation has gained increas-ing attention (Prach & Pysek 1994, 2001; Prach 1994; Prachet al. 2001). It has been claimed that spontaneous successioncan be more efficient than human efforts at returningdegraded lands to their original state and reestablishingthe self-regularity of ecosystems (Prach et al. 2001).Depending on soil conditions, the required time period forestablishment of woody species on degraded mining sites inCentral Europe has been, on average, 20 years (Prach1994). The first individuals of woody species may be presentat the beginning of succession (Rebele 1992; Prach 1994;Prach & Pysek 2001; Rebele & Lehmann 2002), and the

1Institute of Ecology, Tallinn Pedagogical University, 15 Pargi Street, 41537Johvi, Estonia.2Rovaniemi Research Station, Finnish Forest Research Institute, P.O. Box 16,96301 Rovaniemi, Finland.3Department of Botany and Ecology, University of Tartu, 40 Lai Street, 51005Tartu, Estonia.4Department of Mining, Tallinn Technical University, 82 Kopli Street, 10412Tallinn, Estonia.5Address correspondence to M. Pensa, email [email protected]

� 2004 Society for Ecological Restoration International

200 Restoration Ecology Vol. 12 No. 2, pp. 200�206 JUNE 2004

probability of their early establishment is higher in moder-ately wet, nutrient-poor sites (Prach 1994) or in sandy soils(Rebele 1992). In general the establishment of woody spe-cies on degraded lands is highly variable due to the manystochastic factors that affect vegetation succession.

Promoting spontaneous succession for ecological restora-tion therefore requires careful consideration of the targetecosystem and time scale in which the goals of restorationare to be achieved (Prach et al. 2001). If the goal is to restorea certain type of plant community, an engineered restorationprocess is preferred and succession should be directedtoward intended objectives. When selecting suitable treespecies for plantations, variable results rendered by differentspecies under the same environmental conditions must beborne in mind (Singh et al. 2002). For example, Montagniniet al. (1995) have shown that among 20 indigenous treespecies used for reclaiming forests on degraded lands inBrazil, four species tended to have more positive effects onsoil properties than others. On abandoned agricultural landsin India restored with different bamboo species, the diversityof ground vegetation was greatest under the canopy of aspecific species (Arunachalam & Arunachalam 2002).

In this study we selected three tree species, Betulapendula (silver birch), Alnus glutinosa (European blackalder), and Pinus sylvestris (Scots pine), growing in30-year-old plantations established on calcareous spoils ofopencast oil shale mines in Estonia, and evaluated theirimpact on understory and ground vegetation. Reclamationof the Estonian oil shale opencast mining areas is regu-lated by the Ministry of Environment. It has been deter-mined that the goals of reclamation should encompassaspects of both economic (forestry) and ecological impor-tance. However, studies carried out on the reclaimed oilshale opencast mining areas have mainly focused on ques-tions related to forest management (Kaar et al. 1971; Kaar2002). The outcome of reclamation has been measured inthe volume increment of growing stock in stands. Exceptfor pedogenesis (Reintam & Kaar 1999; Reintam 2001;Reintam et al. 2002), other ecological aspects of reclama-tion have been scarcely studied on the oil shale opencastmining areas. Laasimer (1973) and Reintam and Kaar(2002) studied spontaneous succession in the oil shaleopencasts, but they also emphasized the economic out-come, suggesting establishment of plantations as the onlyway to achieve rapid recovery of vegetation on minedareas. If spontaneous succession is permitted, however,valuable ecosystems can develop over decades, whichmay serve as refuges for rare species and communities(Kirmer & Mahn 2001). This proposition is supported bythe results of Holl (2002), who demonstrated that sponta-neous succession can create diverse plant communities onan abandoned coal mine in the United States. The need forcomparisons between engineered reclamation and sponta-neous succession has also been emphasized by Prach et al.(2001) and Pysek et al. (2001). Therefore, we additionallysampled mine spoils where spontaneous succession hasoccurred since 1950.

Three specific questions are addressed by this study: (1)are there differences in characteristics of the tree layerbetween plantations and spontaneous stands; (2) does bio-mass and diversity of ground vegetation in plantations differfrom that of spontaneously developed stands; and (3) havetree species used to establish plantations had differenteffects on ground vegetation biomass and diversity.

Methods

Study Area

The study was conducted at Kuttejou (59�200 N, 26�590 E) andNarva (59�180 N, 27�450 E) oil shale opencast mining areas innortheastern Estonia. Mean annual temperature is 4.5�C inboth areas; annual precipitation is 600mm in Kuttejou and660mm in Narva. Oil shale is the main mineral resource ofEstonia, and it is mostly used for energy production. Miningfor oil shale began at the beginning of the twentieth century,but its extraction and consumption was most intensive in the1980s, declining sharply after the collapse of the Soviet Unionin 1991. A reclamation program for opencast oil shale mineswas initiated in the late 1960s when large areas around Narvawere opened to mining activities. As of 2003, about 9000ha ofoil shale opencast mines have been reclaimed by leveling themine spoils and planting a variety of tree species on them(Reintamet al. 2001).Themost frequently planted tree specieshave beenPinus sylvestris (covering approximately 85�90%ofthe reclaimed area) and Betula pendula. In total 54 specieshave been used for reclamation, but most of them have onlybeen planted experimentally on small plots (Kaar et al. 1971).The reclamation program did not set targets for restoring theopencast spoils inKuttejou,whereminingactivities had ceasedby the 1950s. Since then, theKuttejoumining area (200ha) hasbeen revegetated by means of spontaneous succession to asparse forest with an average stand age of 30 years.

Background of the Mining Areas

Kuttejou opencast mine operated on former agriculturallands from 1925 to 1947. Oil shale layers were mined byhand, and the overburden was stripped by a 3-m3 steamexcavator equipped with a spoiler. The width of the miningpit was 20m, forming spoils with maximum slopes of 20�.The thickness of the oil shale layer was 2.6m, and theoverburden thickness varied from 5 to 12m, with an aver-age stripping factor of 1.9m3/t. Consequently, the height ofthe ground in the mined out area is 0�2m below theoriginal ground surface. As a spoiler and mechanical shovelwere used for stripping, the estimated swell factor ofthe material was 1.3. Maximum size of the material minedwas 1.4m due to bucket size, while the average sizewas 0.15m. Due to the lack of reclamation requirementsand poor mobility of the spoiler, the spoil depth variesfrom 0.5 to 2m. The organic matter content of the spoil,which originates from kukersite oil shale, is on average4�6% and is evenly distributed in the spoil.

Vegetation Restoration on Opencast Oil Shale Mines

JUNE 2004 Restoration Ecology 201

Mining activities at Narva opencast mine began in 1963,and the mine is still operating today. The area was pre-viously covered with swamp forests and bogs. The oil shalelayers are mined by blasting; the overburden is blasted andstripped by 15m3 and 90m�long boom draglines. Thewidths of the mining pits are 50m, forming spoils withmaximum slopes of 3�. The thickness of the oil shalelayer is 2.0m, and the overburden thickness varies from11 to 17m, with an average stripping factor of 4.1m3/t.Consequently, the average height of the ground in themined out area is 2�4m above the original ground surface.The higher surface level and water pumping, which isrequired for opencast operations, results in lower wateravailability and spoil seepage than at the Kuttejou mine.The Narva mining area has been a test site for a variety ofnew stripping methods. As a result, in some areas, the peatlayer is located at the bottom of the spoil and in otherareas, on the top. The organic component of the remainingoil shale layers is mostly at the bottom of the spoil. Drag-line, and in some cases a mechanical shovel, was used forstripping the blasted overburden rock, and therefore theswell factor of the material is 1.4. Maximum size of thematerial mined was 3.1m, while the average size was 0.3m.Spoil depth is uneven and ranges from 0.5 to 1.5m, but canbe as much as 6m in test sections. The organic mattercontent of the spoil, which also originates from kukersiteoil shale, is on average 2% (4% at the bottom and 1% atthe surface of the spoil). Due to the stripping method used,the material is unevenly distributed in the spoil and theestimated swell factor is up to 1.5 at the bottom. This is anadditional factor that causes water runoff from the spoil.The spoil chemistry at the Narva mining area was ana-

lyzed by Vaus (1970) before reclamation (i.e., before treegrowth had influenced pedogenesis). Similar data were notavailable for the Kuttejou mine; therefore we took spoilsamples in the most recently abandoned parts of the open-cast. The samples were analyzed for pH, total nitrogen,and P2O5, and compared with those taken at Narva(Table 1). It must, however, be recognized that the spoilchemistry at Kuttejou has been affected by vegetationestablishment over decades and the results do notrepresent the pre-successional conditions in this area.

Sampling

Four different types of forest stands were sampled: (1) anatural stand formed by spontaneous succession, (2) a

plantation of Pinus sylvestris (referred to as pine stand),(3) a plantation of Betula pendula (birch stand), and (4) aplantation of Alnus glutinosa (alder stand). In June 2002we established five 103 10m randomly located sampleplots in each stand type (20 plots total). According toforest inventory data the stands were on average 30 yearsold, which was confirmed by taking tree cores with anincrement borer from within each plot.

Within the plots we counted all trees with diameter atbreast height (dbh) greater than 10 cm and estimated den-sity of understory (number of shrubs and tree saplings/100m2). For analyzing ground vegetation we marked fivesmall (0.43 0.5m) randomly located quadrats within eachplot. Thus, ground cover in each stand type was rep-resented by 25 quadrats (100 quadrats total). We harvestedall vascular plants from within the quadrats and dried themat 60�C for 48 hr. We determined total aboveground bio-mass and biomass by species for each stand type (g/m2).We also calculated mean number of species, mean speciesdiversity (H) expressed by Shannon�Wiener’s indexweighted with biomass and volume stock of trees. Shannon�Wiener’s index of diversity was calculated as follows:

H ¼Xn

i¼1

pi log2ðpiÞ��

��

where n is the number of species and pi is the proportion ofthe ith species in total biomass.

Data Analysis

To compare the effects of restoration technique (spontan-eous succession vs. plantations) and tree species on char-acteristics of the understory and ground vegetation andtree layer, we applied univariate ANOVA using standtype as a fixed factor. Post hoc comparisons were carriedout using the LSD test. The assumptions of normality andhomogeneity of variances were checked using the Kolmo-gorov�Smirnov D-statistic and the Brown�Forsythe tests,respectively. All statistical computations were made withthe aid of the computer package Statistica’98 (StatSoftInc., Tulsa, OK, U.S.A.) at the significance level a5 0.05(Statistica 1998).

Results

The density of trees with dbh greater than 10 cm wassignificantly lower in the natural stands than in the planta-tions, whereas species composition of the natural standswas more diverse compared with the plantations (Tables 2& 3). There was one dominant tree species (abundance.90%) in all three plantations and a few co-dominantspecies with abundance above 5% (Table 3). Unlike theplantations, no one species dominated in the tree layer ofthe natural stands. Although nearly half of the trees werebirches, which is the most common species in secondary

Table1. Characteristics of spoil chemistry in studied areas

(mean±SD).

Variable Narva n Kuttejou n

pHKCl 7.1 ± 0.2 18 7.4 ± 0.1 6Total N (%) 0.1 ± 0.05 15 0.2 ± 0.06 6P2O5 (mg/kg) 16.6 ± 10.7 18 20.7 ± 8.1 6

Data on Narva were published by Vaus (1970).


202 Restoration Ecology JUNE 2004

succession in oldfields in Estonia, other species were alsofrequently represented (Table 3). The sparse tree layer innatural stands had a positive effect on the density of theunderstory (Table 2). In other stand types there were fewerthan 50 saplings/100m2, the only exception being a plot ina birch stand where Caragana arborescens had beendensely planted (300 saplings/100m2) under the tree layer.This is why mean understory density was high in the birchstands (Table 2); however, the birch stands also had thehighest degree of variation, with densities ranging from 20to 300 saplings/100m2. Species richness of the understoryfollowed the same pattern as that of the tree layer with thenatural stands having significantly more species per plotthan the plantations (Table 2). Although trees were moresparsely distributed in the natural stands, the difference involume stock between the plantations and natural standswas not as clear. When compared with pine stands, naturalstands would provide similar timber production, but whencompared with birch and alder stands, the potential fortimber production was much less (Table 2).

In alder stands, the biomass of ground vegetation wassignificantly greater than that in the other two plantationsas well as in the natural stands (Table 2). Species richnesswas significantly higher in the alder stands than that in thepine and birch stands, where it averaged 7 species/m2

(Table 2). In natural stands, species richness was highlyvariable and did not differ clearly from that of any otherstand type (Table 2). Shannon�Wiener’s diversity indexwas smallest in the birch stands, while among other standtypes its variation was not significant (Table 2). Calama-grostis arundinacea (red grass) alone accounted for morethan half the cumulative biomass in birch stands, while fivespecies in natural, four in alder, and two in pine standsaccounted for more than half the cumulative biomass(Table 3).

Discussion

The studied sites had different historical backgrounds thatmight have influenced the measured characteristics ofvegetation. Mining activity at the Kuttejou mine ended20 years earlier than at Narva; therefore Kuttejou was

exposed longer to species immigration. A longer immigra-tion period was found to enhance biodiversity in studies ofisland colonization (Begon et al. 1996). Differences invegetation between Narva and Kuttejou opencast mines,however, cannot be explained merely by species immigra-tion. First, although less exposed to immigration, theground vegetation in alder stands had the same averagespecies richness as that of natural stands. Second, althoughthe Kuttejou opencast mine was older than Narva, theaverage age of the tree layer was 30 years in both areas.Thus, there was a time lag of about 20 years in the devel-opment of woody stands at the Kuttejou mine. A similartime lag was characteristic of the formation of a tree layerby spontaneous succession in lignite opencast mines inCentral Europe (Prach & Pysek 1994, 2001; Prach 1994).

The second aspect to be considered when comparingstudy areas is their different locations; the Kuttejou mineis situated at about 50 km west of the Narva mine.Although this distance does not result in climatic variation,it may affect propagule availability due to differences insurrounding plant communities. The Kuttejou mine islocated in a diverse agricultural landscape (fields inter-spersed with small areas of forests and gardens), whilethe Narva mine is surrounded by natural ecosystems(swampy deciduous forests interspersed with pine bog for-ests). A more diverse landscape is probably the mainreason why ground vegetation at Kuttejou opencast hasthe greater total number of plant species, but spontaneoussuccession seems to be a necessary condition for maintain-ing that diversity. Although Betula pendula was the mostabundant tree species in the natural stands, its dominancedid not exceeded 50% and groups of other woody specieswere interspersed within gaps. The stochastic nature ofspontaneous succession is thought to be the main reasonfor such spatial patterns (Rebele 1992). This variabilitycreates the required conditions for development of forestunderstory (shrubs and tree saplings) and ground vegeta-tion. Sparse tree distribution was the main reason for thesmall volume of growing stock in the natural stands, indi-cating that their economic value is relatively low comparedwith plantations. In the plantations, contrary to the naturalstands, the tree layer was dense and dominance of the

Table 2. Comparison of the vegetative characteristics among the studied stand types (mean±SD).

Natural Pine Birch Alder

Number of tree species/100m2 3± 1* 1 ± 0.4† 2 ± 1† 1± 0.5†Number of trees/100m2 9± 4* 25 ± 11† 28± 2† 26 ± 7†Volume stock of trees (m3/100m2) 1.0 ± 0.6* 1.9 ± 1.1*‡ 3.4 ± 0.8† 2.8 ± 0.5†‡Number of understory species/100m2 9± 3* 5 ± 2† 5± 1† 3± 2†Understory density (saplings/100m2) 98± 54 38 ± 17 85± 120 16 ± 19Biomass of ground vegetation (g/m2) 52.9 ± 20.8* 32.4 ± 13.7† 59.2 ± 23.6* 104.0 ± 41.3‡Number of species in ground vegetation/m2 12.4 ± 6.9*† 7.0 ± 2.5* 7.0 ± 1.9* 12.2 ± 2.8†Diversity of ground vegetation, H 1.73 ± 0.44* 1.52 ± 0.32* 0.86 ± 0.47† 1.56 ± 0.46*

Values within rows with different symbols are significantly different at a5 0.05. Due to high degree of variation, the significance of differences was not estimated fordensity of understory.



Table3. List of vascular plants in the forest stands growing on the opencast oil shale mines in Estonia.

Stand TypeSpecies Natural Pine Birch Alder

Total number 54 27 28 36

Tree layer 5 2 4 3Alnus glutinosa � � � 91.5*Alnus incana � � � 6.9*Betula pendula 47.7 � 90.8* 1.5Larix decidua � 8.8* � �Malus domestica 2.3 � � �Pinus sylvestris 4.5 91.2* 6.3* �Populus tremula 29.5 � 1.4 �Salix caprea 15.9 � 1.4 �

Understory 11 10 9 7Alnus glutinosa � � � 10.8*Alnus incana � � � 2.7Betula pendula 15.4 44.5 17.5* 5.4Caragana arborescens � � 71.1* �Frangula alnus 1.9 0.6 0.2 5.4Larix decidua � 1.3* � �Padus avium 4.9 0.6 � �Picea abies 2.3 12.9 3.6 13.5Pinus sylvestris 0.9 18.1* 0.7 �Populus tremula 15.5 1.3 0.5 �Rhamnus cathartica 3.5 � � �Ribes alpinum 17.6 � � �Ribes nigrum � � � 54.1Rosa vosagiaca � 1.3 � �Salix sp. 5.4 11.0 4.7 8.1Sorbus aucuparia 26.2 8.4 1.2 �Viburnum opulus 4.4 � 0.5 �

Ground vegetation 41 17 18 29Calamagrostis arundinacea � 24.0 66.0 13.4Calamagrostis epigeios 9.5 � � �Dactylis glomerata 1.3 � � �Deschampsia caespitosa 6.1 0.5 � �Epilobium angustifolium 4.7 0.5 0.5 7.5Epipactis helleborine 0.4 � � �Eupatorium cannabinum � � � 7.6Festuca gigantea 0.3 � � 10.9Filipendula ulmaria 1.8 � � �Fragaria vesca 14.0 13.6 7.6 7.0Geum rivale 4.6 � � �Geum urbanum � � � 3.3Hieracium pilosella 4.2 � � �Medicago lupulina 6.0 � � �Mycelis muralis 1.1 2.0 0.4 0.2Orthilia secunda � 34.2 11.7 �Phragmites communis � � 1.1 �Poa pratensis 1.7 � � �Pyrola rotundifolia 8.6 3.8 0.1 �Rubus idaeus 2.5 � 3.2 2.8Rubus saxatilis 16.7 4.6 1.2 2.0Solanum dulcamara � � � 2.6Solidago virgaurea � 2.1 � �Stellaria media � � � 2.9Taraxacum officinale 0.7 5.3 1.1 3.2Tussilago farfara 2.3 5.1 5.3 0.6Urtica dioica � � � 24.7Valeriana officinalis 1.0 � � 0.4Veronica chamaedrys 2.2 � � �

(continued)



planted species was very high (.90%). As previous studies(Kaar et al. 1971; Kaar 2002) have shown, environmentalconditions on the oil shale opencast spoil are ideal forgrowth of Pinus sylvestris and Betula pendula as well asAlnus glutinosa. Thus, dense stands of these species sup-press the growth of other trees, which otherwise couldspontaneously establish on mined areas.

Although in some cases establishment of plantations ondegraded lands may promote growth of other species(Lugo 1997; Singh et al. 2002), our results indicate thaton calcareous and stony spoils of the opencast oil shalemines, spontaneous succession may have several advan-tages in terms of increased plant diversity. This increasein plant diversity may result in an increase in the diversityof other organisms, as variability in stand structure andspecies composition creates habitats for a variety of organ-isms. Our results also revealed differences in the ability ofthe tree species to modify growing conditions for otherplants as measured by the effect of woody species onground vegetation. Alnus glutinosa, a species that formssymbiotic relationships with nitrogen-fixing bacteria fromthe genus Frankia (Wall & Huss-Danell 1997; Sprent &Parsons 2000), promotes the most ground vegetation growth(Table 2). Vigorously growing herbaceous vegetation inturn suppresses woody seedlings, which seems to be themain reason why the understory density was lowest in thealder stands. Low-ground vegetation biomass in the pinestands is probably related to poor soil-forming capabilitiesof coniferous monocultures as shown by Kilian (1998).

The greatest biomass of ground vegetation observed inthe alder stands was related to high species richness.Although mean species richness in the natural stands wasalso 12 species/m2, it varied widely due to the high spatialheterogeneity of the stand structure. The variation in spe-cies richness of the ground vegetation among plots in thenatural stands, however, followed the same pattern as inforest plantations. In plots with high dominance of Pinussylvestris, the number of species was low, while within theplots where deciduous tree species dominated, speciesrichness of the ground vegetation was higher. Similarresults were obtained by Pitkanen (1998), who found thatthe number of coniferous and broad-leaved tree specieshad a significant effect on the diversity of vegetation inmanaged boreal forests. The high dominance of Betulapendula in the tree layer was accompanied by highdominance of Calamagrostis arundinacea in the ground

vegetation. This was obviously the main reason for thelow-diversity index of the ground vegetation in the birchstands, while in the other stand types no one speciesaccounted for 50% or more of the total abovegroundbiomass (Table 3).

We can conclude that spontaneous succession is a usefultechnique for restoration of small areas of the calcareousand stony spoils of opencast mines and may replace thetypical reclamation technique of planting tree monocul-tures where diversity is the goal. Spontaneous successionenhances establishment of diverse vegetation and maytherefore create habitats for a wide range of organisms.If economic value is set as a priority, however, creation ofplantations or scattered colonization foci may assist inovercoming dispersal barriers and direct succession towarddefined targets (Robinson & Handel 2000). Selection oftree species for planting has a significant impact on devel-opment of the rest of the plant community. Among activereclamation practices in oil shale opencast mines, plantingof Alnus glutinosa gave the best results in terms of totalnumber of vascular species and aboveground biomass ofground vegetation. Planting density may also affect thedevelopment of vegetation in plantations. Further studiesare needed to reveal whether the within-stand variabilityin diversity is connected to the density of the tree layer andwhether there are temporal fluctuations in diversity andbiomass of ground vegetation in oil shale opencast mines.

Acknowledgments

We are grateful to Elga and Ene Rull who helped us with thefield sampling. Sarah Wilkinson and Michael Dunderdalechecked the English language of the manuscript. Thecompany Estonian Oil Shale is thanked for the permission tocarry out the study in the Narva opencast. Plant BiochemistryLaboratory of the Estonian Agricultural University analyzedthe chemistry of spoil samples. Prof. Karel Prach and twoanonymous reviewers made valuable comments and annota-tions on themanuscript. The studywas funded by theMinistryof Education, Republic of Estonia (project no. 0282119s02).

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Table 3. Continued

Stand TypeSpecies Natural Pine Birch Alder

Vicia cracca 3.0 � � �Other 1.2 2.2 0.2 0.3

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